matrix: fix Concat.index -- should be (item, {concat_items})
matrix: fix Concat.index -- should be (item, {concat_items})

Previously, it just returned {concat_items} which isn't right.

Signed-off-by: Samuel Just <16ccadb3f2bf0703b4be974f8606e7e98c1cda29@redhat.com>

ivotron/teuthology,caibo2014/teuthology,zhouyuan/teuthology,dmick/teuthology,ktdreyer/teuthology,SUSE/teuthology,SUSE/teuthology,michaelsevilla/teuthology,ivotron/teuthology,michaelsevilla/teuthology,ceph/teuthology,dreamhost/teuthology,t-miyamae/teuthology,ktdreyer/teuthology,ceph/teuthology,caibo2014/teuthology,robbat2/teuthology,robbat2/teuthology,dmick/teuthology,dmick/teuthology,t-miyamae/teuthology,SUSE/teuthology,zhouyuan/teuthology,dreamhost/teuthology